In a wavelength multiplexing communication system, conventionally, a thermo-optic type variable optical attenuator utilizing thermo-optic (TO) effect is known as an optical attenuator for regulating signal intensity with respect to each wavelength on use. This thermo-optic type variable optical attenuator changes refractive index distributions of an optical waveguide core and its circumferential cladding layer while heating a part of optical waveguide by a heater. Consequently, the optical attenuator attenuates intensity of light to be propagated.
As such “optical waveguide” type devices, there are known variable one variable optical attenuator adopting a thermo-optic method shown in Patent Citations 1 and 2 listed below, for attenuation directly and another variable optical attenuator utilizing interference of lights shown in Patent Citation 3.
The variable optical attenuator of Patent Citations 1 and 2 is constructed so as to directly diffuse or attenuate light by changing a refractive index of the circumference of an optical waveguide core by heat. In addition, the variable optical attenuator of Patent Citations 1 and 2 have an optical waveguide composed of a diffuse part for diffusing input light, an attenuating part for attenuating the light diffused by the diffuse part and a recess for attenuating the light furthermore. With this constitution, the optical attenuator has an asymmetrical-profiled optical waveguide throughout input and output. In the optical attenuator like this, mainly, the recess serves to attenuate light thereby enhancing the attenuator's attenuating effect for light. Referring to FIG. 1 etc. of Patent Citation 1, it is noted that the variable optical attenuator is provided, on both sides of the optical waveguide in a width direction thereof, with three heaters (corres. heaters 5, 6 and 7 in Patent Citation 1). These heaters are adapted so as to apply heat on both sides of the optical waveguide, thereby producing changes in the refractive index of the optical attenuator.
While, the variable optical attenuator of Patent Citation 3 has a function of attenuating light due to the changes in the refractive index by heating, as similar to Patent Citations 1 and 2. In this variable optical attenuator, input light is diverged to two routes of lights and thereupon, one route is heated to change a phase of the relevant light. Thereafter, when the lights in two routes are combined into one again, there is produced an optical interference due to this phase change. The variable optical attenuator can regulate the intensity of output light through the use of optical interference. Again, the variable optical attenuator of Patent Citation 3 comprises two optical attenuators connected in series, each of which utilizes a Mach-Zehnder interferometer. In the variable optical attenuator, there are both established a length of the optical waveguide connecting one optical attenuator with the other optical attenuator and a birefringence, so that its transmittance against a certain polarization component is maximized by the front optical attenuator, while the transmittance against another polarization component is minimized by the rear optical attenuator.    Patent Citation 1: Japanese Patent Laid-open Publication No. 2004-325601    Patent Citation 2: Japanese Patent Laid-open Publication No. 2004-325602    Patent Citation 3: Japanese Patent Laid-open Publication No. 2005-195840